The steady and transient photoconductivity, and related phenomena in the neutron irradiated Si

J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06

The steady and transient photoconductivity, and related

phenomena in the neutron irradiated Si.

J.Vaitkus, E.Gaubas, A.Kadys, V.Kalendra, V.Kazukauskas, A.Mekys, J.Storasta, E.Zasinas

Vilnius University, Institute of Materials Science and Applied Research


Outline:

• A few general words about the photoconductivity & related transport phenomena

• The results:– general data– the peculiarities

• Attempts to conclude


Definition:

If the sample is uniform, then the chosen effects: photoconductivity, light induced transient gratings, Hall and photo-Hall effects, magnetoresistance effects

allow to characterize the behavior of free carriers and the local levels in the sample and their parameters .

The inhomogegenities differently influence these effects, therefore the complex application gives a possibility to recognize what happens in the sample after a certain treatment.


PhotoconductivityPhotoconductivity spectra allows to identify the deep level and

characterize a role of electron-phonon interaction in its environment.

0,5 1,0 1,5

0,0

0,4

0,8

1,2

1

Pho

ton

cros

s-se

ctio

n, a

.u.

Photon energy, eV

41

2

3

-15 -10 -5 0 5 10 15 20

0

400

800

1200

1600

2000

En

erg

y, a

.u.

Q, config. coordinate

ET

ET EOpt

EOpt

E

V

E

M

E

C

The ionizing energy of deep centre in thermal and optical experiments are different


Spectra in irradiated Si


Transitions

EC

EV

ER

ET

0 1000 2000 3000 4000

1E-14

1E-13

1E-12

1E-11

1E-10

2007.05.16T=~15,73KU= -1V1,0eV1mm

abs

I (A

)

t (s)

-1Vset

light on0,05mm 0,1mm 0,2mm light off

0V set


Intrinsic photoconductivity

Where jp is the photocurrent, jp is the photocurrent independent on the light aborption coefficient, s – surface recombination velocity, p free carrier lifetime. This expression that can be transformed into the relation

A=b+ck.

s

According to the classical theory of photoconductivity spectral dependence (T.Smith. Semiconductors. 1959)

pp

p

p

p

kLL

s

j

j

1

11

Recombination rate


Si 3 1016 cm-2 and summary

1. Recombination at the surface mush faster;

2. Induced PC from deep levels ~0.5 and 0.62-4 eV


Steady state lifetime dependence on fluence

The lifetime extracted from the peak photocurrent dependence on the irradiation by neutrons depends as a square root from the fluence.

It can be proposed that it means that the lifetime depends on the distance between the defects in the plane. (50 V bias).


Extrinsic photoconductivity

1012 1013 1014 1015 1016

10-10

10-9

10-8

10-7

10-6

10-5

photoresponse @1200 meV @ 18 Kinverse peak value of photo-conductivity inverse lifetime by MW

I, A

;

othe

r da

ta,

rel.u

.

Fluence, n/cm2

photoresponse @1200 meV @ 18 Kinverse peak value of photo-conductivity - effect of saturation of the level 0.82 eV inverse photoresponse value (steady state lifetime) (~to RC1 concentration)

inverse lifetime by MW (~to RC2 concentration)

At high centre concentration the two step PC can appear, then signal becomes dependent on the lifetime


Transient photoconductivity (TP)

• Light pulse excitation • TP can be measured by:

– DC circuit (contact problems)

– Microwave technique– Free carrier absorption (~ non-sensitive to mobility)

– Transient gratings: measure free carrier concentration profile amplitude, i.e. allows to measure concentration decay law and diffusitivity, separately.

• TP gives information about recombination cannels and traps

Depends on free carrier concentration, mobility, internal electric field distribution


Transient gratings

Two features:

1. The decrease of generated by the light pulse carrier concentration with the fluence.2. The decay of the signal may consist two versions:

1. A simple decay with one time constant; 2. The decay can be separated into the two components (fast a few ns and a “slow”

– tens of ns or more)

I sample

I-1

I

I+1

I0


Transient gratings

0.0 2.0x1014 4.0x1014 6.0x1014 8.0x1014 1.0x1015

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

CE CH linear fit linear fit Confidence limits

1 /

sqrt

Fluence, cm-2

~0.7

The recombination channel which causes the capture time less than 20 ps.

We plan direct measurement this trapping with fs pulses

A peculiarity of this capture:These levels (we propose – the disordered region of cluster) it becomes filled and the remaining e-h pairs participate in the recombination and ambipolar diffusion.


Transport phenomena: Hall, photo-Hall, magnetoresistance effects

• Comparison Hall and magnetoresistance effects – a role of inhomogeneity

• Temperature dependence of mobility – depends on scattering mechanism

tNtvStH

0

11

1

0

0000 1

111

H

H

H U

U

vU

wBE

tvSNNSSN

1

01

H

H

U

UtY

stssH N

n

n

Y

N

nZvA

n

Y

~2 00


Hall mobility and magnetoresistance vs temperature


Hall mobility, annealing, excitation effects

200 250 300 3500

100

200

300

400

500

initial

4 min 800C

+26 min 800C

+24h 800C

H (c

m2 /V

s)

T (K)

2 4 6 8 10 12 14 16

10-3

10-2

10-1

J (cm-2s-1)

2,83*1023

1,22*1023

7,04*1022

3,03*1022

1,40*1022

6,02*1021

3,72*1021

1,71*1021

7,38*1020

2,54 s

JV61AK

(

-1cm

-1)

t (s)


Conductivity vs T


Photoconductivity decay


Compare the dependences on fluence the data measured by photoconductivity and by

microwaves technique

The lifetime measured by microwave technique linearly depends on the fluence while from the photoconductivity – as a square root of fluence!!! It shows that the recombination process is more complicated but the MW measurements were performed at room temperature, and PC – at 18 K (no retraping).

1012 1013 1014 1015 1016

106

107

108

1/,

1/s

Fluence, cm-2

2.4 1023 cm-2

7.4 1020cm-2


Conclusions:

1. Spectral dependences,different transient phenomena reveals the details of the roles of radiation induced changes in semiconductor.

2. It seems that clusters can be seen by a specific trapping and by the appearance of percolation.

3. A lot of work ahead …


Thanks for Your attention !!!


• The peculiarities of dark conductivity, photoconductivity and other transport phenomena were investigated in the neutron irradiated Si. The photoconductivity mechanism, observed deep levels and the effects related to the nano- and micro- defects are discussed.

• At low temperature (18K)• The measurement pf photoconductivity spectral dependence in the high absorption region shows the decrease of

photoconductivity related with an increase of recombination at the surface, but the comparison with a standard surface velocity model show that the main recombination is going not at the surface, but in the layer near to surface. Probably there is different doping of the bulk and the layer near to surface.

• The photoconductivity at the absorption edge linearly depends on the intensity of light that shows that in the investigated range of intensities on type of recombination canters plays the main role. The steady state lifetime depends as a square root on the fluence. The microwave date showed its linear dependence, therefore it is necessary to predict the more complicated process of recombination. It could be proposed that this peculiarity is related with the very fast recombination process observed in the measurements with the picosecond resolution.

• The extrinsic photoconductivity shows two different regions: on – clearly expressed impurity photoconductivity band from the level at 0,8-0,9 eV that at low fluence linearly depends on the fluence and saturate at high fluence. The second band (level ~0,5-0,6 eV) is caused by the induced photoconductivity by capture of carriers generated from the deeper level or the intrinsic photogeneration. These levels can be related with clusters because they demonstrate the accumulation effect that cause the partial compensation of the deeper level (the impurity photoconductivity becomes linear to the excitation).

• Near to the room temperature and down to the liquid nitrogen temperature.• The measurement of Hall effect mobility by Hall effect and by magnetoresistance shows more or less the same

data in the lowest fluence sample. For higher fluences the difference becomes high that can be explained only by model of inhomogeneous sample and percolation type of conductivity. The data of photo-Hall effect support this model. The heating cycle up to 80 C shows the significant change of the percolation and the important role of the overlap of the space charge regions in the bulk of sample. The clusters with the effective charge of a few electron charge was evaluated.


0 100 200 300 4001.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18E

, e

V

T, K

Si

Si bandgap f(T)_


-1000 0 1000 2000 3000 4000 5000 6000 7000-1.00E-013

0.00E+000

1.00E-013

2.00E-013

3.00E-013

4.00E-013

5.00E-013

6.00E-013

7.00E-013

ab

s I (

A)

t, s

2007.05.16T=~15,75 KU= -1V0,6eV1mm

-1V set

light on0,05mm

0,1mm 0,2mm

light off

0V set

0 2000 4000 6000 8000 10000

1E-13

1E-12

1E-11

2007.05.16T=~15,7076KU= -50V0,6eV1mm

abs

I (A

)t (s)

-50Vset

light on0,05mm 0,1mm 0,2mm

light off

0V set


Dark current vs T

0.004 0.006 0.008 0.010 0.012 0.014

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Si 8556-01-65

3 1016 cm-2 nI,

A

1/T, K-1

G Linear Fit of temp001_G

0.581 eV

0,003 0,004 0,005 0,006 0,007 0,008 0,00910-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

Si 8556-01-51

1015 cm-2 n

I, A

1/T, K

abs I (A) Linear Fit of Temp001_G

0.605 eV

Documents

The steady and transient photoconductivity, and related phenomena in the neutron irradiated Si